The setUseFocusableTips(boolean) method causes actual tool tips to be focusable; it doesn't have an effect on the code completion popups. There's currently no way to select more than one choice in the completions list.

I've never seen code completion that lets you select more than one choice, in any editor. Can you give me an example of one that does? Or why this feature would be useful?

I see now that it could be useful for languages like SQL, where you can select multiple specific columns. For most languages, I do not think multiple selection makes sense for code completion, which is why I didn't see the use case.